Automatic protection switching (APS) is a service restoration capability of network elements (nodes) in an optical transport network for protecting against line and path failures. APS is part of the SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) optical transport standards. Traditionally, a line comprises one or more SONET (or SDH) sections or spans between SONET (or SDH) network elements, and a path is a logical connection between those points where service traffic is added to and dropped from the transport network. In general, the network elements supporting APS can detect a line or path failure and transfer affected traffic to a redundant link.
The SONET and SDH standards specify various protection schemes according to which network elements can perform line and path protection. Such protection schemes depend upon the various transport network architectures in which the schemes are operating, such as linear networks and ring networks (virtual line-switched ring or VLSR, unidirectional path-switched ring or UPSR, two- and four-fiber bi-directional line switched rings or BLSR). Linear protection schemes include 1+1, 1:1, and 1:N. For 1+1 and 1:1 protection schemes, one protection path serves to protect one working path or one standby link protects one working link.
In a 1+1 protection scheme, for example, the head-end network element permanently bridges the head-end signal to both working and protection equipment to transmit identical traffic to respective working and protection equipment of the tail-end network element. The tail-end network element continuously monitors both the working and protection signals for failures and automatically selects the protection signal in the event of an optical fiber or node failure. Switches can occur automatically because of line conditions, e.g., loss-of-signal (LOS) and loss-of-frame (LOF), or through user-initiated commands, e.g., manual switch, forced switch, and lockout of protection. In a 1:N protection scheme, a single optical protection path or section protects any one of N working paths or sections.
For Ethernet services, e.g., Gigabit Ethernet, the protection provided within the boundaries of an optical transport network, however, often does not extend beyond its edges, where the optical transport network handoffs Ethernet service traffic to data equipment, such as an Ethernet switch. Rather, in most Ethernet network configurations, the Ethernet service traffic passes over a single link from the tail-end network element to the data equipment, because Ethernet links between such devices are not redundant. Hence, this single link represents, at the physical layer, an undesirable single point of failure for Ethernet services.